late 14c., "proof that something is true," from Old French demonstration or directly from Latin demonstrationem (nominative demonstratio), noun of action from past participle stem of demonstrare "to point out, indicate, demonstrate," figuratively, "to prove, establish," from de- "entirely" (see de-) + monstrare "to point out, show," from monstrum "divine omen, wonder" (see monster). Meaning "public show of feeling," usually with a mass meeting and a procession, is from 1839. Related: Demonstrational.